Method for introducing fluorine into an aromatic ring

ABSTRACT

A process for introducing a fluorine atom into an aromatic hydrocarbon by effecting a substitution reaction between an aromatic hydrocarbon and an NF 4   +   cation containing salt.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

This is a division of application Ser. No. 343,033, filed Jan. 27, 1982,now U.S. Pat. No. 4,423,260.

FIELD OF THE INVENTION

This invention relates to fluorocarbons and to a novel method for theirsynthesis. In a more particular aspect, this invention concerns itselfwith a novel method for introducing a fluorine atom into an aromaticring.

Aromatic fluorocarbons are a well known class of chemical compounds thatfind wide utility for a variety of industrial applications and in thefabrication of various commercial products. They are useful as solvents,electrical fluids, heat transfer fluids and as components in themanufacture of resins, waxes, greases and oils. However, presently knownmethods for synthesizing such compounds by introducing a fluorine atominto an aromatic ring structure are severely limited.

The classic Balz-Schiemann reaction, for example, and methods such asthe decarboxylation of fluroformates are useful for the introduction ofa single fluorine atom, but are generally less useful for multiplefluorine substitution. The use of elexental fluorine or electrochemicalfluorination methods result mainly in addition and not in substitution.Halogen fluorides, such as ClF₃, BrF₃, or IF₅, produce, in addition tofluorine substituted compounds, large amounts of the correspondinghalogen substituted compounds and also some addition products. The yieldof substitution products obtainable with halogen fluorides can beimproved by the use of strong Lewis acids. However, the extremereactivity of the resulting compounds, such as ClF₂ ⁺ BF₄ ⁻ or ClF₂ ⁺SbF₆ ⁻, makes control of their reactions with organic compoundsextremely difficult and unsafe. The utilization of transition metalfluorides, such as CoF₃ or CeF₄ results in addition and saturation,requiring subsequent rearomatisation. Therefore, this method is limitedto highly or perhalogenated aromatics. Pyrolysis of aliphaticfluorocarbons, such as CFBr₃, can also produce fluoroaromatics. However,this method is limited again to the synthesis of perfluorinatedaromatics. Halogen exchange reactions, such as Cl versus F, using HF,alkali metal or metal fluorides are useful, but are restricted tosystems strongly activated towards nucleophilic attack by fluoride ion.Hypofluorites, such as CF₃ OF, are useful for electrophilic andphotolytic fluorinations. The electrophilic fluorinations are limitedagain to activated aromatics, whereas the free radical photolyticfluorinations often lack selectivity resulting in --OCF₃ substitutedby-products and side chain fluorination. The xenon fluorides andespecially XeF₂ are promising reagents for electrophilic aromaticsubstitution, but the full extent of their usefulness is still unknown.The limited availability of xenon, its high price, and the treacherousexplosiveness of their hydrolysis product, Xe_(O) ₃, are drawbackscurtailing its extensive use.

The above listing of some of the known methods of preparing aromaticfluorine compounds, although not extensive, clearly illustrates theproblems prevalent in this area of technology and points out the needfor a reliable, readily available and economically feasible reagent foraccomplishing the electrophilic fluorine substitution of aromatic ringcompounds. Therefore, a research effort was undertaken in an attempt tosatisfy the need for a generally usable reagent.

In theory, the ideal reagent for electrophilic substitution would be asalt containing the F⁺ cation. Unfortunately, such salts do not exist.As an alternative, salts containing complex fluoro cations of the typeXF.sub.(n+1)⁺ could be used. However, to be a strong electrophile, sucha cation should possess high electronegativity. Since highlyelectronegative fluorine compounds generally are very strong oxidizers,most of these cations react too violently with organic compounds to beof practical interest. As a consequence, the research effort referred toabove proved to be unsuccessful. Additional research, however, proved tobe fruitful and culminated in the discovery that the NF₄ ⁺ cationconstitutes an exception to the general rule that such cations react tooviolently with organic compounds. As a result of the present invention,therefore, it was found that aromatic ring compounds, such as benzene,toluene, and nitrobenzene, interact rapidly with NF₄ BF₄ in anhydrous HFto give, almost exclusively, fluorine substituted aromatic derivatives.

SUMMARY OF THE INVENTION

The present invention concerns itself with a method for introducingfluorine into an aromatic ring structure by using NF₄ BF₄ as a reactionreagent. The introduction is accomplished by an electrophilicsubstitution reaction in which up to five hydrogen atoms in the aromaticring can be substituted by fluorine atoms. The reaction can be carriedout by either adding the aromatic compound, such as benzene in vaporform, to a cooled solution of NF₄ BF₄ in HF or, alternatively, by addingslowly a solution of NF₄ BF₄ to a solution of benzene in HF.

Accordingly, the primary object of this invention is to provide a novelxethod for introducing a fluorine atom into an aromatic ring structure.

Another object of this invention is to provide a method for substitutingfluorine atoms for the hydrogen atoms in an aromatic ring structure.

Still another object of this invention is to provide for the synthesisof fluorine containing aromatic ring compounds by effecting a reactionbetween a non-fluorine containing aromatic compound and salts containingan NF₄ ⁺ cation.

A further object of this invention is to provide a method forintroducing a fluorine atom into an aromatic ring by using NF₄ BF₄ as areaction reagent in the electrophilic substitution of a fluorine atomfor a hydrogen atom in an aromatic ring structure.

The above and still further objects and advantages of the presentinvention will become more readily apparent upon consideration of thefollowing detailed description thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Pursuant to above-defined objects, the present invention concerns itselfwith a novel method for introducing fluorine atoms into an aromatic ringcompound through the electrophilic substitution of a hydrogen atom by afluorine atom. The known methods for introducing fluorine into anaromatic ring are quite limited and are often not generally applicable.A widely applicable reagent for carrying out electrophilic substitutionreactions on aromatic ring systems, therefore, would be highlydesirable. As a result, a concentrated research effort was undertakenbased on the hypothesis that the use of NF₄ ⁺ ion containing salts inthis regard would be promising. A continued investigation of aromatichydrocarbon reactions with NF₄ ⁺ ion containing species confirmed thehypothesis. It was found that a reaction between an aromatic hydrocarbonring compound, such as benzene, toluene or nitrobenzene with an NF₄ ⁺salt accomplished the substitution of up to five hydrogen atoms in thearomatic ring by fluorine atoms.

Hydrogen fluoride was used as a solvent because of the high solubilityof NF₄ ⁺ salts in it and also because the diluent and heat dissipationproperties of a solvated system were found to be beneficial in theanticipated vigorous fluorination. As stated hereinbefore, the reactionwas carried out by either adding benzene vapor to a cooled solution ofNF₄ BF₄ in HF or by adding slowly a solution of NF₄ BF₄ to a solution ofbenzene in HF. On contact gas evolution was noted. When rapid additionoccurred some apparent charring occurred. The stepwise substitution of Hby F was observed according to the following general equation: ##STR1##The evolved gas was removed under vacuum and trapped at -196° C. It wasfound to be NF₃ and the amount corresponded to that expected on thebasis of one mole of NF₃ per mole of NF₄ BF₄. Hexafluorobenzene was notobserved although all other substitution products frommono-to-penta-fluorobenzene were obtained. Almost no saturated orpartially saturated fluorocarbons were produced which makes this processof special interest in generating aromatic fluorocarbons directly fromtheir hydrocarbon analogues.

The benzene, toluene, and nitrobenzene reactants interacted rapidly withNF₄ BF₄ in anhydrous HF to give, almost exclusively, fluorinesubstituted aromatic derivatives. With benzene, up to five hydrogenswere replaced, while a maximum of four hydrogens were displaced in C₆ H₅CH₃ and C₆ H₅ NO₂. The direction of the substitution in C₆ H₅ CH₃ and C₆H₅ NO₂ and the lack of side chain fluorination in C₆ H₅ CH₃ support anelectrophilic substitution mechanism when using NF₄ BF₄ as a reactant.Although highly electronegative fluorine compounds generally are verystrong oxidizers, most cations react too violently with organiccompounds to be of practical interest. The NF₄ ⁺ cation, however, wasfound to be an exception. It combines high electronegativity (oxidationstate of +V) with high kinetic stability (it is isoelectronic with CF₄),and its reactions require significant activation energies. Furthermore,NF₄ ⁺ salts, such as NF₄ BF₄, offer the advantage of generating in anelectrophilic aromatic substitution reaction only by-products, such asNF₃ and BF₃, which are unreactive toward the organic compounds. In viewof these prcperties and its ready availability, NF₄ BF₄ was found to bean ideal candidate for electrophilic aromatic substitution reactions. Avigorous ring hydrogen substitition occurred even at -7.8° C. in HFsolution.

In carrying out the reactions of this invention, the nonvolatilematerials were manipulated in a well-passivated (with ClF₃) stainlesssteel vacuum line equipped with Teflon FEP U traps, 316 stainless steelbellows seal valves and a Heise Bourdon tube-type pressure gauge.Hydrogen fluoride work was carried out in an all Monel and Teflon vacuumsystem. Transfers outside the vacuum line were carried out in a drybox.Infrared spectra were obtained using 5 cm path stainless steel cellswith AgCl windows and a PE Model 283 spectrophotometer. Mass spectrawere measured with an EA1 Quad 300 quadrupole spectrometer and ¹⁹ F and¹ H nmr spectra were determined with a Varian EH390 spectrometeroperating at 84.6 and 90 MHz, using CFCl₃ or TMS as internal standards,respectively. Positive chemical shifts are upfield from CFC₃ anddownfield from TMS. Raman spectra were recorded on a Cary Model 83 usingthe 4880 Å exciting line. Gas chromatographic data were obtained using aVarian Aerograph GC with a thermal conductivity detector underisothermal conditions (135 °) with a stainless steel column (1/8"×10')packed with Poropak PS. For the GC determination of the quantitativecomposition of mixtures, uncorrected peak areas were used since responsefactors were not available for all compounds. The solid NF₄ BF₄ wasprepared from NF₃ --F₂ --BF₃ at low temperature using UV activation,which gives analytically pure material.

The simplest aromatic hydrocarbon studied in the previously referred toresearch effort was benzene. With NF₄ ⁺ substrate mole ratios of aboutthree, up to five hydrogens were substituted by F as shown in thefollowing generalized equation. ##STR2## However, at these higher NF₄ ⁺to substrate ratios, the reaction was more difficult to control and more"char" formation was noted. Hexafluorobenzene was not observed as aproduct. If significant amounts had been formed, it would have easilybeen detected by mass spectroscopy since its base peak is the parention. Only trace quantities of partially saturated species, C₆ F₆ H₂ andC₆ F₇ H were observed, indicating that very little addition occurred.

In order to determine the nature of the reaction, two substitutedbenzenes, C₆ H₅ CH₃ and C₆ H₅ NO₂ were also studied. These were chosenfor their well known ability to differentiate between an electrophilicand a free radical reaction path, based on the observed ortho-meta-paraproduct distribution.

In the toluene reaction, the ratio of NF₄ ⁺ to toluene was in the range2-4:1. Thus, an excess of fluorine was available (assuming one F/NF₄ ⁺is available for substitution) and multisubstitution was expected. Theresult of a very rapid reaction is illustrated by the followingequation: ##STR3##

The mass spectra of the products strongly indicate that no side chainfluorination had occurred, in agreement with other spectroscopicevidence. Typical isomer distributions for the ring substitution were:o-F(15), m-F(8), p-F(15), 2,4 di-F(30), and mixed di- and tri-F(25).Obviously, o- and p- products predominate for this electron rich ring, aresult which is compatible with an electrophilic substitution process.

For the nitrobenzene reaction, a 3:10 mole ratio of NF₄ BF₄ F: substratewas used. Even under these conditions, this reaction was less vigorousthan those of benzene or toluene, as exemplified by a slightly slowerNF₃ evolution and the lack of "darkening" of the solution until themixture was finally warmed to about 0°. Fluorine substitution occurredto give C₆ F_(n) H_(5-n) NO₂ (where n=1-4) compounds. Minor amounts ofFNO₂ (HF)_(n) were formed and traces of C₆ F_(n) H_(6-n) species wereobserved, but overwhelmingly the NO₂ group was not displaced. Theobserved products were mainly monosubstituted with the following isomerdistribution: o-F(16), m-F(62), and p-F(7).

The observation of predominantly ortho and para substitution and thelack of side chain fluorination in toluene, and the meta substitution innitrobenzene establishes these NF₄ BF₄ reactions as electrophilicsubstitutions.

For nitrobenzene, the yield of fluorinated products was not determineddue to separation problems caused by the low volatility of the productsand the large excess of nitrobenzene used. However, in view of the highrelative amount of mono-F species, and the limited amount of charring,it is estimated that the yield of substituted products was high. For themuch more reactive C₆ H₆ and C₆ H₅ CH₃, yields varied widely. Volatile,fluorinated species were observed equivalent to 30-60% of the aromaticstarting compounds.

The following examples are presented in order to point out the inventionwith greater detail. The examples, however, are illustrative only andare not to be construed as limiting the invention in any way.

EXAMPLE I

C₆ H₅ NO₂. To a stirred solution of C₆ H₅ NO₂ (10 mmol) in 5 ml HF at-78° was added dropwise over 30 min. a solution of NF₄ BF₄ (2.88 mmol)in 5 ml HF. Reaction of the NF₄ BF₄ was shown by an increase in pressuredue to NF₃ evolution. When all the NF₄ BF₄ had been added, the reactionwas gradually warmed to 0° C. and left overnight. During the warming,the reaction solution changed from pale yellow to dark brown. Keepingthe reaction ampoule at -45°, the NF₃, HF, and other volatile materialswere pumped away through -78° and -196° C. traps. After the majority ofthe HF was removed, the reactor was maintained at 0° C. The materialpassing the -78° C. fraction consisted of a few droplets of a liquidwith a low vapor pressure at ambient temperature. Mass spectroscopy ofthe vapor from the drops showed minor amounts of aromatic fluorocarbonswhich did not contain NO₂ substituents. These were of the empiricalformula C₆ F_(n) H_(6-n) (n=1-4). The principal ion peaks observed werem/e (assign.): 85(NO₂ F HF), 49 (NOF), and 30(NO). Examination of theliquid non-volatiles at 0° C. which remained in the reactor, by NMRspectroscopy, showed that five fluorinated compounds were present andall were found to be substituted nitrobenzenes by comparison of theobserved chemical shifts with reported values. By measurement of thearea of the resonances the amount of each compound was calculated: o-C₆FH₄ NO₂ (14%), m-C₆ FH₄ NO₂ (62%), p-C₆ FH₄ NO₂ (6%), 2,6-or3,5-difluoronitrobenzene (14%), and 2,4-difluoronitrobenzene. The largeexcess of C₆ H₅ NO₂ C employed, and still present, masked the ¹ Hspectra of these products and thus the ¹⁹ F spectra were relied on foridentification.

EXAMPLE II

C₆ H₅ CH₃. Toluene and NF₄ BF₄ (1:4 molar ratio) were reacted bycondensing the hydrocarbon onto the stirred HF solution of the salt at-78° C. Alternatively, toluene in HF at -78° C. was treated dropwisewith a solution of NF₄ BF₄ (1:2 molar ratio). In either case,instantaneous reaction occurred and the solution became black. Afterwarming to 0° C. for a few hours, these reactions were worked up in theusual manner. Much tar like residue remained in the reactor in eachcase. Infrared spectroscopic examination of the volatile species,trapped at -78° C., showed strong bands near 1500 cm⁻¹ confirming thepresence of aromatic species. Mass spectra of these fractions showed inboth experiments that only aromatic substitution products were present;these were of the empirical formula C₇ F_(n) H_(8-n) (where n=1-4). Thelow intensity of the m/e 69 and 51 peaks indicated the absence of CF₃ orCF₂ H groups in these compounds with the observed intensities of thesepeaks being due to C₄ FH₂ and C₄ H₃ ions. From the reaction using ahigher ratio of NF₄ BF₄ to toluene, a significant amount of C₆ F₄ H₂ wasfound indicating some displacement of CH₃ from the ring. The NMR spectraof these products confirmed that various fluorotoluenes were presentapproximately in the amounts given (%): o-C₆ FH₄ CH₃ (15), p-C₆ FH₄ CH₃(16), m-C₆ FH₄ CH₃ (8), 2,4-difluorotoluene (30), other di- andtri-fluorotoluenes (25), and 2,4,5,6-tetrafluorotoluene(7).

EXAMPLE III

C₆ H₆. Benzene and NF₄ BF₄ were reacted using the same two techniquesdescribed for toluene. It was not possible to prevent charring andblackening of the benzene. Nevertheless, isolation of the volatileproducts condensable at -78° C. and examination of their mass spectrashowed that substantial substitution of H by F had occurred, resultingin the formation of C₆ F_(n) H_(6-n) (n=1-5). C₆ F₆ was not observed andonly minor amounts of the addition products C₆ F₆ H₂ and C₆ F₇ H wereobserved.

In consideration of the aforementioned detailed description, it isobvious that the present invention provides a novel method forsubstituting fluorine atoms for hydrogen atoms in an aromatic ringstructure without affecting saturated or oxidized substituents. Theresults of this invention clearly demonstrates the utilization of NF₄ ⁺ion containing salts as powerful reagents for the introduction offluorine atoms into aromatic rings by electrophilic substitution. Up tofive hydrogens can be substituted in aromatic systems by a rapidsubstitution reaction, found to be highly efficient and relatively safe,before a much slower addition reaction takes over.

This slower fluorine addition reaction was also studied and found toproduce the corresponding cyclo-hexadienes and hexenes. The additionreactions are novel and offer a controlled, high yield path to dieneswhich have previously only been obtained as parts of complex mixtures.

To obtain more data on the reactions of aromatics with NF₄ ⁺ salts, anexamination of aromatic systems, which were already highly fluorinated,was carried out. It was found that these starting materials are moreinert toward the strongly fluorinating NF₄ BF₄, thus allowing bettercontrol of the reaction. Experiments were carried out using tetra-,penta-, and hexafluorobenzene as starting materials. All reactedgradually at, or near, ambient temperature. All solutions and productswere colorless throughout the reactions. Liberated NF₃ and excess,unreacted NF₄ BF₄ were recoverable. The products were identifiedspectroscopically and most of them have been reported in the literature,making their identification unequivooal. The overall results are shownby the equations. ##STR4##

For these three highly fluorinated benzenes, the addition of the firsttwo fluorines occurs in para position to each other (1,4 addition) andortho to any hydrogen, if present. The addition of a second pair offluorines cannot proceed by a 1, 4 mechanism without changing the ringinto a bicyclo form, which is generally encountered only underphotolytic conditions. Thus, the second pair of fluorines undergoes a 1,2 addition to yield a cyclohexene.

For pentafluorobenzene, some substitution was also observed. It is notclear whether this is the result of a true substitution or of aaddition-elimination reaction. In the case of p-C₆ F₄ H₂, the second F₂addition produces the 1H,4H-octafluorocyclohexene which has two possiblegeometric isomers. Trace quantities of the saturated product, C₆ F₁₀ H₂,were also detected by mass spectroscopy.

In order to provide greater detail in connection with the additionreactions referred to above, Examples IV, V and VI are presented. Inthese addition reactions, almost no hydrogen substitutions occurred. Theaddition of the first pair of fluorine atoms always gave1,4-cyclohexadiene in which the CF₂ group was ortho to hydrogen on thering. The addition of the second pair of fluorine atoms results in theformation of cyclohexenes. These reactions occurred in high yield. Allproducts were characterized spectroscopically and by comparison toliterature data.

EXAMPLE IV

C₆ F₆. A sample of NF₄ BF₄ (4.07 mmol) contained in a Teflon (FEP)ampoule was dissolved in anhydrous HF (4 ml) and cooled to -78° C.,Hexafluorobenzene (1.25 mmol) was condensed into the ampoule which wasthen warmed gradually while stirring magnetically. After being keptovernight at 0°-10° C., the clear, colorless solution was cooled to -78°C. and the volatile material quickly removed by condensation into a-196° C. trap. The -196° C. trap contained NF₃ (1.24 mmol) contaminatedwith traces of HF as shown by infrared spectroscopy. The reaction wasallowed to continue for another day at room temperature. Whilemaintaining the reaction ampoule at 0° C., the volatile products and HFwere separated by fractional condensation in a series of U-traps cooledat -45, -78, and -196 . The -196° C. fraction, NF₃ and HF, was discardedand the -45 ° C. trap was empty. The -78° C. trap contained a whitesolid, which melted to a colorless liquid above 0° C. Examination ofthis material by infrared and gas chromatography showed it to be 1,4perfluorocyclohexadiene (1.18 mmol, 94.3% yield, based on C₆ F₆) with aslight amount (2-3%) of unreacted C₆ F₆. Intense ions in the massspectrum were observed at m/e (assign.):224(C₆ F₈), 205(C₆ F₇), 186(C₆F₆) 155(C₅ F₅, base), 136(C₅ F₄), 124(C₄ F₄), 117(C₅ F₃), 105(C₄ F₃),93(C₃ F₃), 86(C₄ F₃), 74(C₃ F₂), 69(CF₃), 55(C₃ F), and 31(CF). The ¹⁹ FNMR spectrum showed two equal area multiplets at 113.1 and 158.3 ppm inagreement with the literature for 1,4-C₆ F₈. A white solid remained inthe reaction ampoule and was shown by Raman spectroscopy to be NF₄ BF₄(1.48 mmol).

EXAMPLE V

C₆ F₅ H. As before, a mixture of NF₄ BF₄ (4.29 mmol) and C₆ F₅ H (1.35mmol) in HF was stirred and warmed during several hours from -78° C.toward ambient temperature where it was kept for 12 hours. Evolved NF₃(3.3 mmol) was monitored. After several more hours of stirring at roomtemperature, the products were separated by vacuum fractionation throughU-traps cooled at -45, -78, and -196° C. All material passed the -45°trap except for a small amount of NF₄ BF₄ remaining in the reactor. The-196° C. fraction was discarded. The -78° C. trap contained 1.24 mmol ofa colorless liquid whose infrared spectrum indicated that it wascomposed of more than one cyclohexene [1170(ms), 1740(s), and 1720 cm⁻¹(vs)], as well as unreacted C₆ F₅ H. Gas chromatography showed threecomponents which were analyzed individually by mass spectroscopy. Inorder of elution they were; (1) 1,4-C₆ F₈, 26.1%, (2)1H-heptafluorocyclohexa-1,4-diene, 66.3%, and (3) C₆ F.sub. H, 7.6% withthe composition based on GC peak areas. The mass spectra of thefractions agreed very well with published data for the assignedcompounds. In addition, the ¹⁹ F NMR spectra confirmed the formulatedstructures. For 1-H heptafluoro-cyclohexa-1,4-diene, a literature reportof the NMR spectrum could not be found, but by comparison with relatedcompounds it was apparent that the observed resonances and area ratioswere reasonable for that structure. Chemical shifts of H or F, ppm or δ(rel. F peak area): 1-H, 5.93; 2-F, 127.7(1); 3-F, 115.2(2); 4-F,160(1); 5-F, 155(1), 6-F, 101.7(2). The conversion of the C₆ F₅ H was92%. The composition of the product was 28% 1,4-C₆ F₈ and 72% 1, 4-C₆ F₇H, with a total of 91% of the organic material being recovered.

EXAMPLE VI

p-C₆ F₄ H₂. A mixture of NF₄ BF(4.18 mmol) and p-C₆ F₄ H₂ (1.43 mmol) in4 ml HF at -78° C. was stirred and warmed to 0°° C. over 3 hours,followed by overnight stirring at 0°-20° C. Fractional condensation at-78° C. and -196° C. was used to separate HF and NF₃ from the productswhich were retained in the -78° C. trap. No unreacted NF₄ BF₄ remainedin the reactor. The original -78-20 C fraction was further separated byrefractionating through -45 and -78° C. traps. The former contained 0.21mmol of a colorless liquid whose infrared spectrum showed a strong bandat 1710 cm⁻¹ typical for the double bond of a --CF═CH--group. Analysisusing GC/MS procedures showed this material to be 1H,4H-hexafluorocyclohexa-1,4 -diene. Prominent mass spectral peaks werefound at m/e (assign.): 188(C₆ F₆ H₂), 169(C₆ F₅ H₂), 150(C₆ F₄ H₂),138(C₅ F₄ H₂), 137(C₅ F₄ H), 119(C₅ F₃ H₂, base), 99(C₅ F₂ H), 94(C₃ F₃H), 93(C₃ F₃), 88(C₄ F₂ H₂), 81(C₅ FH₂), 80(C₅ FH), 75(C₃ F₂ H),69(CF₃), 68(C₄ FH), 61(C₅ H), 57(C₃ FH₂), 56(C₃ FH), 51(CF₂ H), 50(CF₂),44(C₂ FH), and 31(CF). The ¹⁹ F NMR spectrum for this compound agreedwith published data. Similar analysis of the -78 ° C. fraction showed itto be a mixture of unreacted p-C₆ F₄ H₂, the above described 1H,4Hcyclohexa-1,4-diene, and a compound of empirical formula C₆ F.sub. 8 H₂.An infrared spectrum of the latter compound showed bands at cm⁻¹(intens.): 3070(w), 2960(w), 1710(ms), 1380(s), 1355(w), 1260(m),1150(s), 1065(m), 1030(m), 743(mw), 637(w), 580(w), and 582(w). Thebands near 3000 cm⁻¹ are assignable to the carbon-hydrogen stretches of--C═C--H and C--H groups while the 1710 cm⁻¹ peak is typical of a--CF═CH--stretching vibration. Strong ion peaks in the mass spectrumwere at m/e (assign.): 226(C₆ F₈ H₂), 207(C₆ F₇ H₂), 186(C₆ F₆), 157(C₅F₅ H₂) 144(C₄ F₅ H), 137(C₅ F₄ H), 119(C₅ F₃ H₂), 117(C₅ F₃), 113(₄ F₄H), 94(C₃ F₃ H), 93(C₃ F₃), 75(C₃ F₂ H), 69(CF₃), 57(C₃ FH₂), 51(CF₂ H),and 50(CF₂). The NMR spectra of the -78° fraction confirmed the presenceof p-C₆ F₄ H₂, 1H,4H-hexafluorocyclohexa-1,4-diene, and1H,4H-octafluorocyclohexene; chemical shift of H or F,ppm or (rel. area)of 1H,4-C_(6F).sbsb.8 H.sbsb.2:1-H,5.1(1); 2-F, 121.5(1), 3-F, 118.1(2),4-H, 4.7(1), 4-F, 134.4(1), 5-F, 130.3(2), 6-F, 110.4(2). The conversionof starting material was 78%. The composition of the products was 53% C₆F₆ H₂ and 47% C₆ F₈ H₂, with a total of 92% of the organic materialbeing recovered.

As will be clearly evident to those skilled in the art, variousalterations and modifications of the present invention can be madewithout departing from the spirit thereof, since it is intended that theinvention be limited only by the scope of the appended claims.

What is claimed is:

1. A process for adding flourine across the double bonds in a highlyhalogenated aromatic hydrocarbon which comprises the step of effectingan addition reaction between a highly halogenated aromatic hydrocarbonand a hydrogen fluoride solvent solution of NF₄ BF₄.
 2. A process inaccordance with claim 1 wherein said highly halogenated aromatichydrocarbon is tetrafluorobenzene.
 3. A process in accordance with claim1 wherein said highly halogenated aramatic hydrocarbon ispentafluorobenzene.
 4. A process in accordance with claim 1 wherein saidhighly halogenated aromatic hydrocarbon is hexafluorobenzene.